I think the issue is more complicated than most folks are pointing out here. Let me use two bikes I have as a example. My 1972 Bottecchia is 58.5cm C-C. My 2002 Bianchi is 54.5cm C-C. That appears to be a huge difference yet both bikes fit me. To begin with, the Bottecchia is "square", that is 58.5 ST/58.5 TT. The Bianchi is proportional - 54.5 ST/57.0 TT so although the frame on Bianchi is much smaller, the top tube length is only a little shorter. Even that is a bit of an illusion - the bars on the Bottecchia are the classic 'U' shape and the normal riding possition is either on the forward extension of the bars or in the drops on the long flat sections where your hands fit nicely. The bars on the Bianchi have that "anatomic" bend. The two positions that I mentioned as normal for the Bottecchia are unusable on the Bianchi. Instead, you reach forward and ride the hoods of the Ergo levers or, if in the drops, you reach forward to the flat part of the bend in the bar. The horizontal section of the drops where I would ride on the Bottecchia is too short to fit my hands on the Bianchi. Because of having to reach forward, that slight 1.5cm difference in top tube length is cancelled out and you have effectively the same riding position.

If I were to get the next size down Bottecchia, 21" or 53.3cm, it would also have a short 53.3cm top tube which would be much too short. If I put the handlebar and Ergo levers from the Bianchi on my Bottecchia the reach would be too long (unless I got a shorter stem obviously). So essentially you have to look at the modern frame as a package - top tube length, stem, handlebars, integrated brake/shifters, seat post, saddle - to arrive at an appropriate reach and then see what you get for a frame size. And typically it will be shorter than a traditional frame for the same reach.

Assuming the bikes you're talking about are later (mid 80's- early 90's?) then I would bet they use fairly modern dimensions. I'm 5'10" with a 30" inseam and a 56cm might be slightly large for me, depending on the top tube length if you consider that the 54.5cm Bianchi is just right. So if you are 2 inches taller than me with 4 inch longer legs (yeah, I'm shaped like Fred Flinstone) than your upper body is 2 inches shorter than mine but your arms are probably longer. Depending on just how long your arms are compared to mine, you might have the same or up to an inch or so more reach than me or 0-2.5cm more. So in a frame with modern demensions, a 57-59cm top tube should be about right. So that 56cm (if measured C-C) might just work. Try it an see.